(1) Field of the Invention
This invention relates to the drilling of wells into subterranean reservoirs, and more particularly to a composition and method for use thereof in drilling such wells wherein penetration of the reservoir by the composition is minimized, thus minimizing the reduction in the permeability of the reservoir which can result from an invasion of the reservoir by the composition. The invention especially relates to such a composition and method employing a composition which has low fluid loss properties during its use and ultimately substantially dissolves or disperses in the formation fluid with which it comes in contact.
(2) Description of the Prior Art
In various well drilling, completion, treating and workover operations in permeable hydrocarbon-producing reservoirs, it is often advantageous to inject a fluid into the well in such a manner that the fluid is in contact with the reservoir penetrated by the well. The injected fluid may be used, for example, as a drilling fluid, a hydraulic fracturing fluid, an acidizing fluid, a fluid for the placement of a gravel pack in the well and the like. The injected fluid has a tendency to penetrate the reservoir. Since most reservoirs are heterogeneous in permeability at least to some degree, the injected fluid tends to preferentially flow into the zones of higher permeability, called thief zones. Not only does this flow result in a loss and waste of the fluid, but also prevents the injected fluid from entering into zones of lower permeability in substantial quantities, thus causing poor fluid distribution between zones of different permeability. Accordingly, fluid loss control agents, and in particular plugging agents, have been developed for use in such fluids. These fluid loss control agents tend to plate out on the face of the reservoir into which the fluid is being injected and restrict further fluid flow through that portion of the reservoir. In the various well operations, it is necessary that the fluid loss control or plugging agent be eventually readily removed from the hydrocarbon-producing zones to prevent a permanent and an attendant reduction in oil production rate. Removal of the plugging material may be effectively accomplished by utilizing an agent that is soluble in the reservoir fluids, either water or hydrocarbons, and producing the well. However, many of the prior art materials are either insoluble under bottom hole conditions, or are so highly soluble that they are difficult to place in the reservoir before they dissolve and fail to provide and maintain the required plugging action throughout the treating operation. It is therefore essential that the fluid loss or plugging agent composition possess the property of controlled solubility wherein a satisfactory solid plug will be formed for a period of time necessary to carry out the well operation, which plug will thereafter be removed by being slowly dissolved in the reservoir fluids.
It is also advantageous to utilize an additive that is largely soluble in hydrocarbons and insoluble in water, thereby leaving any water-producing strata permanently sealed. Thus, selective plugging is effected, the hydrocarbon-producing strata are temporarily plugged and the water-producing strata remain permanently sealed. On removal of the temporary plugging agent from the hydrocarbon-producing strata, oil and gas production capability is fully restored, while water production is permantly eliminated or substantially decreased.
Various slowly oil-soluble, water-insoluble particulate agents useful in well drilling and treating operations have been developed. U.S. Pat. No. 3,302,719 discloses such solid particles comprised of a homogeneous mixture of a polymer such as a poly-1-olefin or a copolymer of ethylene and an alkyl acrylate, a wax such as a paraffinic petroleum wax, and a resin such as an ester of rosin or an aliphatic hydrocarbon resin. The mixture is added to a pumpable liquid carrier fluid and injected into a well.
U.S. Pat. No. 3,882,029 describes finely divided particles comprising a mixture of a wax, an oil-soluble surface active agent, a water-dispersible surface active agent, an ethylene-vinyl acetate copolymer and a fatty alcohol. The particles are dispersed in an aqueous salt solution containing chrome lignite, hydroxyethylcellulose and xanthan gum.
U.S. Pat. No. 3,954,629 shows finely divided particles comprising a mixture of polyethylene or an ethylene-vinyl acetate copolymer, a polyamide and a softening agent such as a long chain aliphatic diamide or a polyterpene resin. The particles are suspended in a liquid carrier.
In brief, these prior art compositions contain wax which is an inherently soft particle, an agent which hardens the wax particle, for example the widely used ethylene-vinyl acetate copolymer, and one or more additives which have low solubility in oil but are utilized to improve the fluid loss control properties of the particles, for example chrome lignite. In spite of the wide variety of known well treating compositions which compositions have certain advantages, need remains for compositions which have both improved fluid loss control and cause reduced permeability damage to the hydrocarbon-producing zones of the reservoir.
Accordingly, it is a principal object of this invention to provide a well treating composition and method for use thereof which combine improved fluid loss control properties with reduced permeability damage to the reservoir.
Yet another object of the invention is to provide such a composition which contains a minimum of oil-insoluble components.
Still another object of the invention is to provide such a composition which contains solid particulate matter having a variety of particle sizes.
Other objects, advantages, and features of the invention will be apparent from the following description.